In recent years, with changes in demand for services, cellular radio communications systems have been switching from second-generation mobile communication systems (such as Personal Digital Cellular (PDC)) to third-generation mobile communication systems (such as Wideband direct sequence Code Division Multiple Access (W-CDMA)).
Moreover, it is expected that fourth-generation mobile communication systems will be introduced in the future for more sophisticated and diversified services. Also, radio communications systems other than cellular radio communications systems will be more diversified.
Current assignment of frequency bands (frequency channels) to radio communications systems is essentially fixed assignment of a required frequency band to a single radio communications system, so as to avoid inter-system interference with another radio communications system.
However, with future diversification of radio communications systems, it will become difficult to reserve frequency bands, and there is a need for a technology for a plurality of radio communications systems for different uses to share the same frequency band.
Such a technology for sharing will allow flexible and efficient radio communications systems to be developed according to demand of the market and users. For a plurality of radio communications systems to share a frequency band, however, an interference avoiding technology for reducing degradation in communication quality and system capacity will be required.
As an example of using the same frequency band by a plurality of radio communications systems, a mixed communication environment of a wireless LAN radio communications system standardized in IEEE 802.11b and a Bluetooth radio communications system which use an Industrial, Scientific, and Medical (ISM) band of 2400 to 2483.5 MHz as shown in FIGS. 1(a) and 1(b) is known.
As shown in FIG. 1(a), frequency channels used in the wireless LAN system are within a range of 2412 to 2484 MHz and are assigned, overlapping at 5 MHz intervals. On the other hand, frequency channels used in the Bluetooth system are within a range of 2402 to 2480 MHz, and are set without overlapping at 1 MHz intervals.
As shown in FIG. 1(b), in the wireless LAN system, high-speed wireless LAN data with a 1210 μsec length modulated by Direct Sequence Spread Spectrum (DSSS) system is transmitted. In the Bluetooth system, Bluetooth data modulated by Frequency Hopping Spread Spectrum (FHSS) system which randomly changes a transmission frequency within a 79 MHz band every 625 μsec is transmitted.
Therefore, when the wireless LA system and the Bluetooth system are used at the same time, two pieces of Bluetooth data are transmitted while one piece of high-speed wireless LAN data is transmitted. At this time, if a frequency band hopped in the Bluetooth system overlaps a frequency band used by the wireless TAN system as shown in FIG. 1(b), a data collision (mutual interference) occurs between them, causing loss of data.
As measures to avoid such data collisions in radio communications systems, various methods have been proposed. In the wireless LA systems the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) system is currently applied to avoid data collisions. In the Bluetooth systems, the Adaptive Frequency Hopping (AFH) system is used.
In the CSMA/CA system, in addition to carrier sense, a preamble is transmitted before transmitting data from a transmitting terminal to a receiving terminal. Only when there is a response from the receiving terminal, the data is transmitted, so that a data collision with another transmitting terminal can be avoided. In the AFH system, frequency hopping is performed, adaptively avoiding a frequency band in which a data collision will occur, so that mutual interference can be avoided.
As another example of the case where a plurality of radio communications systems are in the same frequency band, a mixed communication environment of an Orthogonal Frequency Division Multiplex/Time Division Multiple Access (OFDM/TDMA) system and a GSM system having compatibility with the OFDM/TDMA system is disclosed in a patent document 1.
Specifically, the patent document 1 discloses a technology in which, as shown in FIG. 2(a), subcarriers in the OFDM/TDMA system are assigned to frequency bands which do not overlap frequency channels in the GSM system, and, as shown in FIG. 2(b), an integer multiple of an OFDM/TDMA slot is made to be equal to one or an integral number of GSM slots, and a pilot symbol is assigned to every (n−1) subcarrier (“n” is an integer more than one), so that occupied bandwidths for carriers in the radio communications systems can be used without overlapping between the radio communications systems.
In a cellular radio communications system in which a limited frequency band is used only by a single radio communications system, control of frequency band assignment at base stations is performed with the impact of interference by channels in the same frequency band in the single radio communications system taken into account.
In conventional Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) radio communications systems, when frequency channels are fixedly assigned for use at base stations, the limit value of the Carrier to Interference power Ratio (CIR) necessary for maintaining a required communication quality is specified, and the repeated use distance of channels in the same frequency band and channels in an adjacent frequency band is determined so that the CIR locational degradation rate is lower than or equal to a predetermined value.
Here, when the repeated use distance is small, channels in the same frequency band can be repeatedly used geographically densely, so that the number of frequency channels available at each base station is increased and the system capacity is increased, while interference by channels in the same frequency band used at another base station increases the deterioration rate of communication quality.
On the other hand, when the repeated use distance is large, the deterioration rate of communication quality can be held down, while the number of frequency channels available at each base station is reduced and the system capacity is reduced.
In conventional FDMA and TDMA radio communications systems, a threshold of the repeated use distance or the amount of interference for guaranteeing communication quality is predetermined, and frequency channel design is performed without exceeding the threshold.
In a conventional single radio communications system, in contrast to the above-described fixed channel assignment, the Dynamic Channel Assignment (DCA) is known which performs dynamic channel assignment in order to increase system capacity and frequency use efficiency.
In the CDMA radio communications systems, repeated arrangement of channels in the same frequency band is theoretically possible. However, when a plurality of microcells using the same frequency bard for communication are located in a macrocell, the DCA is still effective as a measure against interference between channels in the same frequency band in the macrocell.
In such CDMA radio communications systems, a hierarchical cell structure in which microcells are located in a macrocell for example may be adopted. A technology for effectively using a frequency band in the hierarchical cell structure is disclosed in a patent document 2 and a non-patent document 1.
The patent document 2 proposes a method in which, in the case where a macrocell radio communications system and a microcell radio communications system, which are different in transmission speed share the same frequency band, when one radio communications system is short of assignable frequency channels, permission to use is given sequentially from an unused frequency channel of low priority in the other radio communications system and a partition as the boundary between a frequency band in the macrocell and a frequency band in the microcells is shifted. (See FIGS. 3(a) and 3(b)).
In the technology according to the patent document 2, high-priority frequency channels are rearranged during dynamic frequency channel assignment to a macrocell in the macrocell and a plurality of microcells in the macrocell.    Patent document 1: Japanese published unexamined application No. 2000-69575    Patent document 2: Japanese published unexamined application No. H11-205848    Non-patent document 1: Ogura Hirotsugu, “Frequency Channel Assignment Method and Network”
As described above, the conventional radio communications systems have the problem that, when a plurality of radio communications systems use the same frequency band, such as when a wireless LAN system and a Bluetooth system are mixed as shown in FIGS. 1(a) and 1(b), mutual interference between the radio communications systems reduces the communication capacity of the other radio communications systems.
Also, the conventional radio communications systems have the problem that, as shown in the Japanese published unexamined application No. 2002-111631, for example, a frequency band usable in one radio communications system is limited by a securable number of frequency channels in a frequency band used by the other radio communications system.
Also, conventional radio communications systems have the problem that traffic concentration in one radio communications system makes it difficult to secure frequency bands in the other radio communications system, making it impossible to handle uneven traffic distribution.
Generally, in cellular radio communications systems, since mutual interference between channels in the same frequency band degrades communication quality, the amount of interference and the allowable amount of interference (an interference amount threshold set in each radio communications system) on a particular frequency channel are compared to determine whether the frequency channel can be used or not.
Also, in the FDMA and TDMA radio communications systems, the above-described allowable amount of interference is determined to meet a required communication quality, according to various parameters (such as modulation systems and error correction technologies), the number of repeated use of channels in the same frequency band, the type of traffic, and the like in the radio communications systems.
On the other hand, in the CDMA radio communications systems, since spreading gain can be obtained by spreading transmission signals, interference tolerance is large, and repeated use of channels in the same frequency band in a single cell is possible. The allowable amount of interference varies, depending on the variable spreading ratio and transmission power control according to traffic and the type of traffic.
The impact of two radio communications systems having different characteristics to interference when sharing the same frequency band without applying interference avoidance control is shown in FIGS. 4(a) and 4(b).
As shown in FIG. 4(a), the FDMA radio communications system has had a problem that when the amount of interference increases over the allowable amount of interference, frequency channel assignment becomes difficult. In particular, the FDMA radio communications system suffers strong interference, not only in the same cell but also from an adjacent cell, from the CDMA radio communications system which performs repeated use of channels in the same frequency band in a single cell.
Also as shown in FIG. 4(b), the CDMA radio communications system has had a problem that when the amount of interference from the FDMA radio communications system increases, the system capacity is reduced.
For these reasons, it is necessary to assign frequency channels, taking account of interference in the radio communications systems to make such adjustments that the amount of interference is made lower than or equal to the allowable amount of interference for the FDMA radio communications system, and a sufficient system capacity can be secured for the CDMA radio communications system.
In addition, when two radio communications systems use different frequency bandwidths impacts on transmission characteristics as shown in FIGS. 5(a) and 5(b) are generated.
In terms of narrowband signals, as shown in FIG. 5(a), there is a problem that interference exceeding the allowable amount of interference with a number of consecutive narrowband signals is generated, and transmission becomes difficult. In terms of wideband signals, as shown in FIG. 5(b), there is a problem that notches due to narrowband signals occur in a frequency bandwidth used, distorting the signal waveform, and thereby deteriorating transmission quality. Thus, control is required to maintain transmission quality in each radio communications system.